The present invention relates to a mirror with a dielectric coating on a substrate. The invention further relates to an etalon comprising two such plane-parallel mirrors.
Mirrors having a dielectric coating on a substrate are often used in particular when a process involves ultraviolet radiation. The dielectric coating has low refractive index and high refractive index dielectric layers in alternating arrangement. Depending on layer thicknesses, they have particularly high reflectivities at specific wavelengths. High refractive index and low refractive index are to be understood in the present case as the relative refractive index difference at the wavelength at which the respective mirror is used. By way of example, the material of a layer of a dielectric coating that is referred to as low refractive index has a lower refractive index at the wavelength for which the dielectric coating has a particularly high reflectivity than the material of the layer of said dielectric coating that is referred to as high refractive index and is arranged in alternating fashion therewith.
Dielectric mirrors are used, inter alia, in microlithography with ultraviolet radiation, in particular at wavelengths between 157 nm and 365 nm. For example, WO 2010/034367 A1 discloses a dielectric mirror, which has at least one oxidic layer stack of low refractive index and high refractive index oxides and at least one fluoridic layer stack of low refractive index and high refractive index fluorides, wherein at least one mixed layer stack of an alternating sequence of oxidic and fluoridic layers is placed between the oxidic and the fluoridic layer stack. Said dielectric mirror at the same time has a high reflectivity with at the same time low stress induced by the dielectric coating.
A further field of use for mirrors with dielectric coating are lasers, in particular excimer lasers, which emit at wavelengths between 157 nm and 365 nm. The exact wavelength is monitored using Fabry-Pérot interferometry. To this end, two plane-parallel mirrors with dielectric coating are used to form, by way of spacers, an etalon, into which a partial ray of the radiation emitted by the laser is radiated and reflected to and fro between the two mirrors. In dependence on the distance between the two mirrors, the rays emerging from the etalon have different path differences and form different interference patterns. Therefrom, the wavelength of the radiation emitted by the laser can be determined. If it deviates from the predefined wavelength, the laser can be readjusted.